The DC-DC converter supplies a predetermined voltage and a predetermined current to a load by turning on/off one or a plurality of switch elements and controlling on/off time (pulse duty) of a switching pulse.
As this DC-DC converter, a switching DC-DC converter of inductor type having high conversion efficiency is frequently used. In the switching DC-DC converter, a current mode control scheme becomes a mainstream for the reason that the response speed is fine and phase compensation is simple.
For example, in the conventional current mode control DC-DC converter (see, for example, U.S. Pat. No. 6,127,815 and U.S. Pat. No. 6,366,066), a divided voltage obtained by dividing an output voltage by using resistors is input to an inverting input terminal of an error amplifier and a constant reference voltage is input to a non-inverting input terminal of the error amplifier. The pulse duty is controlled by a feedback loop so as to make the divided voltage coincide with the reference voltage. As a result, a desired voltage can be obtained by changing the resistance ratio.
In addition, a filter is connected to the output of the error amplifier, and phase compensation is conducted so as to bring the DC-DC control loop into stable operation.
Among various characteristics of the DC-DC converter, the conversion efficiency is very important. Especially in portable devices, demand for the conversion efficiency is very strict because it is linked directly with a battery life. Therefore, it is demanded that the DC-DC converter operates with low power dissipation as far as possible.
Especially in the DC-DC converter of the current mode control scheme, the switching loss and loss caused by current dissipation becomes dominant in the state of a light load which has little load. Therefore, the DC-DC converter of the current mode control scheme is demanded to operate in “light load mode” with the current dissipation reduced to improve the efficiency.
In the conventional DC-DC converter, the output of the error amplifier which equivalently indicates the peak current of the inductor is monitored by a mode comparator, and the mode is determined to be the light load mode when the output of the error amplifier has become a certain threshold voltage or less (i.e., when the load current has become a certain value or less) (see U.S. Pat. No. 6,127,815 and U.S. Pat. No. 6,366,066 described above).
In the light load mode, all or a part of unnecessary circuits is turned off to suppress the current dissipation and improve the efficiency. The mode comparator has hysteresis to prevent immediate return to a normal mode.
The conventional DC-DC converter has a problem that the current dissipation of the error amplifier cannot be suppressed. For improving the conversion efficiency, it is necessary to reduce the current dissipation of the error amplifier as well in the light load mode.
However, a problem is caused at the time of return to the normal mode.
In other words, the timing of return from the light load mode to the normal mode is when the load current increases and the error amplifier output has exceeded the threshold voltage and the hysteresis of the mode comparator. At this time, the output of the mode comparator is inverted to bring about the normal mode operation, and the error amplifier output arrives at a voltage value depending on a required load current (indictor current), resulting in the stationary state.
If the current dissipation of the error amplifier is decreased in the light load mode (or an error amplifier having low current dissipation is used) to improve the efficiency, a large output voltage drop is caused at the time of return from the light load mode to the normal mode.
This output voltage drop is caused because the error amplifier has a low current and consequently the time for charging up the filter circuit connected to the error amplifier output is prolonged.
In other words, the output voltage (filter voltage) of the error amplifier is proportionate to the peak current of the inductor. The demanded load current cannot be let flow unless this voltage is raised. Therefore, the energy supply becomes insufficient and the output voltage drops greatly.
The great output voltage drop causes false operation in some connected applications. Therefore, the DC-DC converter is demanded to supply a voltage of a desired level stably.
If the error amplifier current is reduced to improve the efficiency, the output voltage drop at the time of mode transition becomes great. On the other hand, if the error amplifier current is increased to avoid the output voltage drop, the efficiency is aggravated.
Therefore, the conventional DC-DC converter has a problem that it becomes difficult to reduce the current dissipation of the error amplifier.